1. Field of the Invention
The present invention relates to a power transmission transformer for a noncontact power transfer device, and especially to a power transmission transformer for transferring electric power to a rechargeable battery used as a power supply for a cordless phone, a portable device and the like, the electric power being transferred in a noncontact state without through a metal contact from a power transmission device to a power receiving device by electromagnetic induction.
2. Related Art
FIG. 8 shows a noncontact power transfer device using a self-oscillation circuit in a power transmission portion as a circuit scheme which has a simplified configuration and low power consumption.
In FIG. 8, a power transmission side 10 which is a power transmission device includes a power transmission transformer T composed of a transmitting coil L1 and a drive coil L2 for self-oscillation, and a switching element Q1 in a power transmission portion. A power receiving side 20 which is a portable device includes a receiving coil L3 and a rectifying and smoothing circuit, and further a charge control circuit and a rechargeable battery not shown. Then, a winding surface of the transmitting coil L1 and a winding surface of the receiving coil L3 are placed to be opposed to each other in separable housings, and electrical power is transferred.
FIG. 9 shows an arrangement in which the power transmission transformer T including the transmitting coil L1 and the drive coil L2 in the power transmission portion of the power transmission side 10, and the receiving coil L3 of the power receiving side 20 are disposed in such a configuration.
Japanese Patent Laid-Open No. 2002-17046 describes a method for transferring electric power to be here described. The transmitting coil L1 is formed of an air-core coil using self-bonding wire. The drive coil L2 is a printed coil formed similarly to a circuit pattern on the surface of the same as a substrate P1 for a peripheral circuit, and a winding surface of the printed coil and a winding surface of the air-core coil which is the transmitting coil are fixed to each other with an adhesive agent or the like across the substrate.
Further, the receiving coil L3 is formed of an air-core coil composed of the same self-bonding wire as that of the transmitting coil, and the air-core coil is attached on a substrate P2, and has a winding surface thereof placed on the bottom surface side of a housing. A winding surface of the receiving coil L3 and a winding surface of the transmitting coil L1 are placed to be opposed to each other across their own housings, and electric power is transferred from the device 10 which is the power transmission side to the device 20 which is the power receiving side.
According to this method, there is a problem that, when an input direct current (DC) voltage is specified to be low, the drive coil L2 formed of the printed coil has a larger loss.
Further, when the drive coil L2 is used in a resonance circuit of a ringing choke converter (RCC) which is another self-oscillation circuit, a drive voltage equal to or larger than a gate threshold voltage of a MOSFET is necessary because the drive coil L2 uses the MOSFET as a switching element, and accordingly a winding number has to be increased to form by using the printed coil, and the printed coil itself then becomes large, so that practical application has been difficult.
Further, if the drive coil L2 is used in a collector resonance circuit, and the transmitting coil L1 is placed on the surface opposed to the printed coil L2 across the circuit substrate, then coupling between the transmitting coil L1 and the drive coil L2 lowers by the amount corresponding to a thickness of the substrate. Thus, it is necessary to increase a winding number for that, and the winding number has to be increased to form by using the printed coil, and the printed coil itself becomes large, so that practical application has been difficult.